Impacts. This study aims to improve Veterans' adherence to evidence-based treatment for PTSD, through increasing family support for treatment. Improving retention rates in evidence-based PTSD treatment will positively impact Veterans' health and well-being, lower the cost of treating PTSD, and decrease long-term demand for PTSD services. If effective, this approach could help resolve national calls for routine inclusion of family involvement in PTSD treatment. Once demonstrated for PTSD, these strategies could be utilized for other conditions and problems relevant to Veteran populations (e.g., suicide prevention, TBI rehabilitation) and stimulate shifts across practice and policy to better routine and evidence-based involvement of families in care. Background. PTSD occurs in as many as 1 in 5 combat Veterans and is associated with a host of negative, long-term consequences to the individual, their families, and society at large. Evidence- based psychotherapies, such as Prolonged Exposure (PE), result in clinically significant symptom relief for many. Yet, adherence to these treatments (i.e., session attendance and homework compliance), which is vital to ensuring recovery, can be poor. Engaging families in Veterans' treatment may provide a powerful method for promoting EBP adherence. Our data indicate that 70% of Veterans express some interest in involving their family in their care for PTSD; yet, only 17% of providers have had any contact with Veterans' families. The objective of the proposed study is to evaluate the effectiveness of improving family support as a tool to improve Veterans' EBP adherence. This research agenda directly addresses two VA HSR&D priorities: 1) innovative mental health care; 2) improving the quality of life for Veterans and their caregivers. The work aligns with the VHA Blueprint for Excellence and Strategic Plan through meeting the unique needs of military-service disabled Veterans, providing a novel treatment approach, and emphasizing patient- and family-centered care. Objectives/Aims. Aim 1: To improve Veterans' adherence to PE through engaging families in care. H1: Veterans randomized to family supported PE will attend more sessions (H1a) and report greater homework compliance (H1b) than Veterans randomized to standard PE delivered in routine care. Aim 2: To improve the clinical outcomes of Veterans receiving PE through engaging families in care. H2: Family supported PE will be more effective than standard PE in reducing PTSD severity and comorbid problems (depression, quality of life, relationship functioning) from baseline to posttreatment. Aim 3: To examine barriers/facilitators of implementing family support for PE. Exploratory Aim: To identify mechanisms underlying adherence differences between treatment conditions. We will explore if adherence differences are mediated by changes on key social influence variables (family perceptions of treatment credibility, family support for PE, and family symptom accommodation). Methods. We are proposing a practical randomized controlled trial to compare Veteran adherence, and to PE with and without family attendance at PE's educational sessions, with the ultimate goal to improve Veterans' clinical outcomes. For Aim 3, we will use a concurrent process evaluation to identify potential implementation facilitators and barriers to family involvement in PE within VA. Participants will include Veterans with clinically significant symptoms of PTSD across three sites, plus a family member or friend of the Veteran. Aim 1 outcome variables include session attendance and homework compliance. Aim 2 outcomes include PTSD symptom severity, depression, quality of life, and relationship functioning, measured monthly over the course of treatment. Key social influences (Exploratory Aim) will be assessed through brief weekly self-reports.